In the last two decades, organic and mineral fibers such as polyvinyl alcohol, nylon, polypropylene, basalt, rayon, cellulose, steel, wood, aramid, polyester, acrylic, and others are able to be spun into very different deniers or dtex numbers and cut to different lengths. These fibers have high fiber strength and those with low dtex or fine deniers also have a great number of fibrils per unit weight that allows very cost effective means of using such fibers. However, such fibers with fine fibrils have a high tendency to form fiber lumps or fiber balls when they are added into cement or concrete slurries in a typical mortar or concrete mixer. Thus very low percentage (<0.3%) of such fibers could be used in these hydraulic materials even with the mixing aids such as fillers or granules, so the full advantage of these fibers can never be realized. In order to increase the amount of fiber loadings in the hydraulic slurries to achieve the desired tensile strength and free of shrinkage cracking high denier and stiffer fibers have thus been tried. However, the significantly reduced number of fibrils per unit weight of these high denier fibers made them very expensive to use and the efficiency is also low relative to those with finer deniers. So a method or process that could allow uniform distribution of fibers of very fine denier and at high percentage is highly desirable for the cement based building products where high tensile strength without shrinkage cracking is desired.
Fly ash is often produced by the combustion of coal. Enormous amounts of fly ash are produced annually worldwide. Disposal of fly ash poses an increasingly difficult problem because the volume, the particulate nature and the varied chemical composition of fly ash. Fly ash of particular composition, such as fly ash F and fly ash C, has been used as an additive in Portland cement to reduce the raw material cost, but often at less than 30%, because such coal burning byproducts are not self-activating and often needs excess lime or other activator when higher amount of fly ash is used in the cement slurry.
U.S. Pat. No. 7,468,154, using high shear mixing, presented a preparation consisted of high polymer containing fly ash C composite that shows high bendability. However, the addition of Portland cement into the fly ash C, as stated in the patent, resulted in shrinkage cracking in a matter of hours after casting. Such high shear blending of a hydraulic slurry shown in this reference or any other conventional mixing techniques for high solid cement or gypsum slurries will not allow greater than 0.3% fiber, especially low dtex fibrils, to be incorporated into the slurry without fiber balling or lump formation. Without sufficient fiber presence, such cement, cement-fly ash blends and composites are crack prone and are unable to reach high modulus of elasticity and/or compressive strength for many building material applications.
Conventional incorporation of foam into a cement or concrete slurry is through foam generators, compressed air and low shear mixing. The high shear mixing described in U.S. Pat. No. 7,468,154 will essentially destroy most of the preformed foam in the slurry during the mixing, and is thus not applicable for making foamed hydraulic composites where preformed foam is used.
It is well known that the presence of polymers or polymer dispersions in the cementitious composites will greatly improve cement hydration in thin cast applications, and the polymer will improve the adhesion in concrete repairs where fresh concrete is applied onto the surface of old concrete. However, the polymer additives in a typical cement dominant composition have very little effect in enhancement of tensile or bending strength due to the inherent brittleness and high crystalline content associated with the typical hydrated cement. U.S. Pat. No. 4,352,856 and U.S. Pat. No. 7,468,154 showed that, using only fly ash and >10% by dry weight of polymers or resins, such cement-free composite could develop good bendability or moderate tensile strength. However, without proper activation of the fly ash by cement or base such low tensile strength still prevents it from being qualified for a lot of building product applications where high modulus of rupture and high compressive strength are required. The high amount of polymer used in U.S. Pat. No. 7,468,154 may increase not only the raw material cost of a cementitious material, but also the flammability of the resulting composites that many cementitious building materials try to avoid. So a lower polymer usage yet being able to achieve high modulus of rupture (MOR) and high compressive strength is highly desirable.
The use of low level of fly ash (<30%) in cement to reduce cost and yet not significantly affecting the rate of cement set and long term compressive strength is well known. In high fly ash (>40%) cement compositions there are references using various additives such as alkali silicate (U.S. Pat. No. 4,696,698, U.S. Pat. No. 7,727,330), alkali carbonate (U.S. Pat. No. 7,727,330), lithium carbonate (U.S. Pat. No. 6,251,178), citrates (U.S. Pat. No. 8,366,823) or others to activate the fly ash and reduce the set time or increase the rate of strength development. But the inherent brittleness, low tensile strength and the lack of good water resistance associated with typical cement and concrete compositions are not addressed in these references.